1. Field of the Invention
The present invention relates to a transfer molding process and an apparatus to carry out the process and, more particularly, to a transfer molding method and apparatus used for mold-sealing semiconductor devices.
2. Description of the Prior Art
Semiconductor devices such as transistors, integrated circuits (IC), and large-scale integrated circuits (LSI) are sealed for protection against changes in temperature, humidity, and other external conditions, and against breakage or changes in inherent characteristics due to mechanical vibration, shock, etc. In current practice, semiconductor devices are hermetically sealed using either metal and ceramic materials or plastic packages composed of thermosetting resins such as epoxy or silicone. Of these, the plastic package method is predominant.
In the plastic package method, a transfer molding apparatus is used. A lead frame bearing a semiconductor chip is placed in a cavity of a mold, then molten resin is charged and injected into the mold under a constant pressure by a plunger operated by a hydraulic device or the like.
During this method, however, the pressure of the resin as it enters the cavity decreases due to leakage or friction between the mold and the plunger. This decrease in pressure results in voids and other defects inside the product and a poor surface quality of the product.
To overcome this problem, conventional practice is to effect the transfer molding manually, i.e., to have an operator control the pressure of the resin to maintain a constant level. This method, however, lowers productivity.